铅箔增感屏之所以有增感作用，是因为X射线与γ射线能从铅中激发出（）来，从而使胶片感光，达到增感的目的。
参考答案：电子
Not until____________the exam _________ the importance of doing homework.
A．had he failed; he realized
B．he had failed; did he realize
C．he had failed; he realized
D．had he failed; did he realize
根据语境，续写句子，构成语意连贯的一段话。（2分）
我该学习小草的刚劲：钻出了泥土，就得忍耐着亢旱或阴湿；我该学习树木的倔强：抵抗着风雨，抵抗着冷热的气候和虫害；我该学习 ； 。
